In today's business and scientific world, color has become essential as a component of communication. Color facilitates the sharing of knowledge and ideas. Companies involved in the development of digital color print engines are continuously looking for ways to improve the total image quality of their products. One of the elements that affects image quality is the ability to consistently produce the same quality image output on a printer from one day to another, from one week to the next, month after month. Users have become accustomed to printers and copiers that produce high quality color and gray-scaled output. Users now expect to be able to reproduce a color image with consistent quality on any compatible marking device, including another device within an organization, a device at home or a device used anywhere else in the world. There has been a long felt commercial need for efficiently maintaining print color predictability, particularly as electronic marketing has placed more importance on the accurate representation of merchandise in illustrative print or display media. If a user is aware of the intended illuminant (daylight, tungsten, florescent, etc.) for a certain output, color accuracy can be improved if the device is tuned for that illuminant.
Description of color, color perception and psychological and physiological phenomena involving light, object and observer, including color measurements using spectrophotometers are described in R. W. G. Hunt, “The Reproduction of Color in Photography, Printing and Television”. Fourth Edition, Fountain Press, Tolworth, England 1987 ISBN 0-8524-2356. It is well known that human perception of color identity can vary with illuminants.
The models presented in this specification use a device independent color space to consistently track a set of target colors. L*, a*, b* are the CIE (Commission Internationale de L'éclairage) color standards utilized in the modeling. L* defines lightness, a* corresponds to the red/green value and b* denotes the amount of yellow/blue, which corresponds to the way people perceive color. A neutral color is a color where a*=b*=0.
Conventional calibration and characterization of a four-color (cyan, magenta, yellow and black) printer or copier involves at least the following processes: (1) generating a 3-D look-up table (LUT) for mapping device independent parameter space to CMY (cyan-magenta-yellow) space; (2) executing a GCR (gray component replacement)/UCR (under color removal) strategy to convert the CMY space parameters to CMYK space parameters which represent the colors of a typical four-color marking device; (3) constructing marking device TRCs (tone reproduction curves) to account for marking device variabilities (normally done at the time of manufacturing or whenever the printer calibration and characterization process is involved); and (4) applying a suitable half-toning strategy to convert the CMYK continuous tone description obtained after using the 3-D LUTs in steps 1 and 2 above and 1-D LUTs in step 3 above, to the image, to a binary description (e.g., bits to be received by a raster output scanner or similar device for outputting the image). The first two steps are generally regarded as part of the printer characterization. The third step is normally called calibration.
Previously known standard approaches for calibrating and characterizing a system for different illuminants comprised building different profiles, i.e., different LUTs and TRCs for different illuminants—a costly, time consuming and expensive endeavor. A large database of LUTs and TRCs compensating for different color correction for each different illuminant, can place prohibitive demands on system storage and increase the complexity of profile management.
The TRCs are stored plots of an input parameter value versus an output parameter value for a particular color. A TRC is a monotonically increasing marking device function in input-output contone space or input-output density space or input-output byte space, or combinations thereof. In other words, a TRC indicates the value of the output parameter for a specific device that must be used to reproduce the input parameter (if the input and output parameters are exactly equal, then the inputs and outputs are expressed in the same coordinate space). Inaccuracies in the TRC construction step can lead to inaccuracies in color balancing and the 3-D LUT.
Obtaining TRCs for a particular color marking engine is a calibration process, which can be constructed by printing predetermined target colors and measuring the printed target colors using a spectrophotometer or insitu color sensors. Predetermined target colors can be printed as a separate calibration sheet, or as chronological jobs in the banner sheet/header sheet or else the target colors can be extracted from the customer image and measured either by measuring straight from the output image or by rendering subsets of customer colors as target color patches in banner or header pages. Using the target colors and their measured counterparts, and by processing the measured colors, TRCs are adjusted on-line at some desired intervals or on request during system or color balance set ups.
One method of obtaining 1-D TRCs is associated with achieving neutral gray balance. Grayness is an indication of how accurate a process color is, compared to its theoretical ideal of zero chroma (that is, a*=0, b*=0). When equal amounts of cyan, magenta and yellow are printed on a white paper, a well balanced printer should produce a neutral gray of the same amount. Instead, other colors, such as a brownish color, rather than a neutral gray may regularly occur. The system will not produce the desired gray due to various limitations on color pigments of the primaries and the internal processes of the print engine. To overcome this effect, gray-balanced TRCs are used as one-dimensional LUTs to modulate the amount of cyan, magenta and yellow proportions depending on the state of the materials and the print engine. The TRCs are obtained by printing a number of patches, mostly near neutral. In the methods practiced by the color reproduction industry, colors are measured using offline spectrophotometers and amounts of cyan, magenta and yellow are then modified, generally, by using model based algorithms to produce the desired gray-balanced TRCs. Sometimes this process of printing and producing TRCs is iterated several times until satisfactory results are obtained.
The subject invention is particularly useful to provide solutions to the foregoing color problems for a wide range of color workflow practices. Printing and product enhancements are provided that would enable customers to manipulate color documents on a screen before even printing/displaying an output on different output devices in ways that improve the accuracy of the output when viewed under any of several illuminants.
The subject invention exploits a key enabling factor for these operational advantages by constructing the TRCs for individual primaries with implementation of a dynamic color balanced control system tuned for different illuminants for automatic calibration of a full color digital printing system.